This invention relates to improvements in a tube-bending machine which performs the bending of a tube or pipe with a mandrel inserted in the tube.
Problems which occur during tube bending with the tube-bending machine of the type specified above includes rupturing of the tube being bent as is ascribable to an insufficient lubrication of the inner surface of the tube, and the appearance of wrinkles on the inner side of the tube bend, the distortion of a tube section or the thinning of the outer side of the tube bend as is attributed to a deviation in the optimum mandrel position.
FIG. 1 is a view for explaining a case of resorting to the radial-draw bending as a typical example of the tube-bending machine (refer to Journal of Mechanical Working Technology, 3 (1979) 151`166). Referring to the figure, a mandrel 1 is inserted into a tube 2, the tube is clamped to a bend die 4 by a clamp die 3, and the bend die 4 is thereafter rotated a predetermined angle, whereby the radial-draw bending machine bends the tube. In the figure, numeral 11 indicates a wiper die and numeral 12 a pressure die, between which the tube 2 is movably supported. With the prior-art tube-bending machine, however, variations in the lubrication condition of the tube and the setting position of the mandrel cannot be detected during the working, and hence, the worked products must be inspected. This leads to the disadvantage of an inferior available percentage of the products.